dt-bindings: pinctrl: stm32: Fix 'st,syscfg' schema

The proper way to add additional contraints to an existing json-schema
is using 'allOf' to reference the base schema. Using just '$ref' doesn't
work. Fix this for the 'st,syscfg' property.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: linux-gpio@vger.kernel.org
Cc: linux-stm32@st-md-mailman.stormreply.com
Cc: linux-arm-kernel@lists.infradead.org
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>